1. Field
The embodiments discussed herein are directed to a communication device using an orthogonal frequency division multiplexing (OFDM (including OFDMA)) (hereinafter called “OFDMA”) method.
2. Description of the Related Arts
Recently, the development of a communication system under the background of a technology standardized by WiMAX (IEEE802.16e) has been promoted. In IEEE802.16e sequence, the power saving before initial ranging is not especially taken into consideration in a network entry operation. In the connection sequence with a base station, a terminal unconditionally starts a network entry operation after the completion of synchronization. Since this network entry operation is started regardless of a band that a terminal after network entry desires to use. Therefore, if the band that is desired to actually use cannot be used after entry, the terminal must either wait for until the band becomes able to be used or be disconnected. If the terminal is disconnected, power used in the initial ranging operation so far performed is wasted. If origination and disconnection are repeated until the desired band can be secured, the resources of the base station must also share the load.
FIG. 1 explains the problem of the present WiMAX.
It is assumed that terminals 2˜n are accommodated in the base station A and its radio resources are fully used. Since the radio resources of the base station A are overcrowded, terminal 1 cannot communicate with the base station A even if terminal 1 enters from the outside and attempts to communicate with the base station A. However, according to the present WiMAX specification, terminal 1 performs all the network operations for the base station A and after entry terminal 1 checks whether it can use a desired band in order to communicate with the base station A. Therefore, if the desired band cannot be used, terminal 1 must either abandon communications with the base station A or wait for until the desired band can become able to be used. However, all the network entry operations must be performed in order to determine whether terminal 1 can communicate with the base station A. Therefore, if the communication is impossible, the network entry operations are wasted to lead to extra power consumption.
FIG. 2 explains the prior art.
Patent Document 1 discloses a technology for reducing a time needed from origination to connection by transmitting information known by the base station side to a terminal side and Patent Document 2 discloses a technology for regularly receiving information (used throttle information) from the base station and using the information as the trigger of terminal origination on the terminal side. However, in order to realize this, in either method the base station side have to not only have a mechanism for transmitting its intra-office state to the terminal side and also the terminal and base station sides share network entry sequence.
FIG. 3 is a sequence chart of the network entry sequence.
The base station transmits a frame towards terminals in its own cell every 5 msec. First, a down link (DL) frame is transmitted. Upon receipt of the DL frame, a terminal performs synchronization (synchronization between the base station and the terminal) to enable the decoding of its frame data (1). Then, an initial ranging process is performed between the terminal and the base station (2). This is the adjustment control process of transmitting power and the like between the base station and the terminal. Then, basic capability is obtained between the base station and the terminal (3). Specifically, the capability of a physical layer (transmitting power, FEC, etc.) is exchanged with each other between the base station and the terminal. Then, each of the base station and the terminal runs an authorization sequence (4) in order to decode data transmitted from the opposite party to exchange their decoding keys. Then, the terminal notifies the base station of its terminal MAC layer-connected function and it is notified of a formal connection ID by the base station (5). Then, the terminal obtains an IP address (6) and performs service flow control, such as connection control, QOS control and the like, in order to for exchange user data (7) and starts a communication service.
If it is determined in the service flow control that the base station and a terminal cannot communicate with each other, a communication service cannot be provided. Therefore, the operations (1)˜(7) are wasted and extra power is consumed.
Patent Document 1: Japanese Patent Application Publication No. 2006-332753
Patent Document 2: Japanese Patent Application Publication No. H11-252663
However, if the origination itself of the terminal side is controlled by information provided by the base station side, it is necessary for the base station side to monitor the number of terminals and the number of available bands for each OFDMA frame and to store the information in the management area of the OFDMA frame. It is also necessary to extract the information for each frame on the terminal side, to run network entry sequence until receiving a frame from the base station and to synchronize the base station side service with the terminal side service. Although for this purpose it is necessary to unify these services between a different base station vender and a different terminal vender, there are many technical problems in synchronizing this specification and mounting them although they are already technically standardized.